Scalable, real-time messaging system

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving from a plurality of publisher clients a plurality of messages, assigning each of the messages to one of a plurality of distinct channels wherein each channel comprises an ordered plurality of messages, storing messages of each of the channels in one or more respective buffers according to the order, each buffer having a respective time-to-live, retrieving messages for one or more of the channels from respective buffers having time-to-lives that have not expired and according to the order, and sending the retrieved messages to a plurality of subscriber clients.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority from U.S.patent application Ser. No. 14/821,416, filed Aug. 7, 2015, entitled“Scalable, Real-Time Messaging System.” The preceding patent applicationis hereby incorporated by reference in its entirety.

BACKGROUND

This specification relates to a data communication system and, inparticular, a system that implements real-time, scalablepublish-subscribe messaging.

The publish-subscribe pattern (or “PubSub”) is a data communicationmessaging arrangement implemented by software systems where so-calledpublishers publish messages to topics and so-called subscribers receivethe messages pertaining to particular topics that are subscribed to.There can be one or more publishers per topic and publishers generallyhave no knowledge of what subscribers, if any, will receive thepublished messages. Some PubSub systems do not cache messages or havesmall caches meaning that subscribers may not receive messages that werepublished before the time of subscription to a particular topic. PubSubsystems can be susceptible to performance instability during surges ofmessage publications or as the number of subscribers to a particulartopic increases.

SUMMARY

In general, one aspect of the subject matter described in thisspecification can be embodied in methods that include the actions ofreceiving from a plurality of publisher clients a plurality of messages,assigning each of the messages to one of a plurality of distinctchannels wherein each channel comprises an ordered plurality ofmessages, storing messages of each of the channels in one or morerespective buffers according to the order, each buffer having arespective time-to-live, retrieving messages for one or more of thechannels from respective buffers having time-to-lives that have notexpired and according to the order, and sending the retrieved messagesto a plurality of subscriber clients. Other embodiments of this aspectinclude corresponding systems, apparatus, and computer programs.

These and other aspects can optionally include one or more of thefollowing features. Storing messages of a particular channel in one ormore respective buffers can comprise storing messages of the particularchannel earlier in the order in buffers having time-to-lives that canexpire sooner than message buffers used to store messages of theparticular channel later in the order. Storing messages of a particularchannel can comprise identifying a first buffer for storing messages ofthe particular channel, arranging messages for the particular channelbased on, at least in part, a respective time each of the messages wasreceived from respective publisher clients, and storing a first portionof the messages of the particular channel as arranged in the firstbuffer. The arrangement of messages for the particular channel cancomprise the order of those messages for the particular channel. Theaspect can further comprise determining that the first buffer is fulland, based thereon: identifying a second buffer, and storing a differentsecond portion of the messages of the particular channel as arranged inthe second buffer. Storing the first portion of the messages can furthercomprise buffering one or more of the first portion of the messages in alocal data buffer, and storing the buffered messages in the first bufferafter a number of buffered messages exceeds a predetermined size orafter a predetermined time period has elapsed. Retrieving messages for aparticular channel and sending the retrieved message to a plurality ofsubscriber clients can further comprise retrieving messages for theparticular channel and storing the retrieved messages in a local buffer,wherein each message can be removed from the local buffer when the arespective time of publication of the message has exceeded apredetermined time period, and sending the retrieved messages to aplurality of subscriber clients from the local buffer according to theorder. A particular channel can be initiated by a message from apublisher client. Retrieving messages for a particular channel can beinitiated by a connection from a subscriber client subscribing to theparticular channel. Each channel can be qualified by a respective namespace. Each message buffer can store messages for a single channel. Eachbuffer for a particular channel can expire at a different time based onthe time-to-live for the buffer. Each buffer of the plurality of bufferscan be assigned to a respective node. At least two respective nodes canbe different physical computing devices. A particular buffer assigned toa particular node can correspond to a computing process on theparticular node. Each buffer for a particular channel can furthercomprise one or more blocks, each block storing one or more messages ofthe particular channel and having a respective time-to-live. Retrievingmessages for the particular channel can further comprise retrievingmessages stored in one or more of the blocks having respectivetime-to-lives that have not expired.

Particular embodiments of the subject matter described in thisspecification can be implemented to realize one or more of the followingadvantages. In a messaging system, a multiplexer node receives messagesfrom multiple publisher clients and assigns each message to one ofmultiple distinct channels of the messaging system. Each channel of themessaging system comprises an ordered sequence of messages. Themultiplexer node stores messages of a channel in buffers that each has arespective time-to-live, e.g., a limited and often short lifetime. Themultiplexer also retrieves messages for one or more channels fromrespective buffers having time-to-live that have not expired. Themultiplexer then provides the retrieved messages to multiplesubscribers. In this way, the multiplexer node can multiplex and mergerequests of client devices subscribing or publishing to the samechannel, thus representing multiple client devices as one, instead ofone by one. Furthermore, since the retrieved messages of a channel arearranged in the same order as the messages are stored in buffers for thechannel, different subscriber clients can receive the channel's messagein the same order.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example system that supports the PubSubcommunication pattern.

FIG. 1B illustrates functional layers of software on an example clientdevice.

FIG. 2 is a diagram of an example messaging system.

FIG. 3A is a data flow diagram of an example method for writing data toa streamlet.

FIG. 3B is a data flow diagram of an example method for reading datafrom a streamlet.

FIG. 4A is a data flow diagram of an example method for publishingmessages to a channel of a messaging system.

FIG. 4B is a data flow diagram of an example method for subscribing to achannel of a messaging system.

FIG. 4C is an example data structure for storing messages of a channelof a messaging system.

FIG. 5 is a flowchart of an example method for publishing andsubscribing messages for publishers and subscribers of a messagingsystem.

DETAILED DESCRIPTION

FIG. 1A illustrates an example system 100 that supports the PubSubcommunication pattern. Publisher clients (e.g., Publisher 1) can publishmessages to named channels (e.g., “Channel 1”) by way of the system 100.A message can comprise any type of information including one or more ofthe following: text, image content, sound content, multimedia content,video content, binary data, and so on. Other types of message data arepossible. Subscriber clients (e.g., Subscriber 2) can subscribe to anamed channel using the system 100 and start receiving messages whichoccur after the subscription request or from a given position (e.g., amessage number or time offset). A client can be both a publisher and asubscriber.

Depending on the configuration, a PubSub system can be categorized asfollows:

-   -   One to One (1:1). In this configuration there is one publisher        and one subscriber per channel. A typical use case is private        messaging.    -   One to Many (1:N). In this configuration there is one publisher        and multiple subscribers per channel. Typical use cases are        broadcasting messages (e.g., stock prices).    -   Many to Many (M:N). In this configuration there are many        publishers publishing to a single channel. The messages are then        delivered to multiple subscribers. Typical use cases are map        applications.

There is no separate operation needed to create a named channel. Achannel is created implicitly when the channel is subscribed to or whena message is published to the channel. In some implementations, channelnames can be qualified by a name space. A name space comprises one ormore channel names. Different name spaces can have the same channelnames without causing ambiguity. The name space name can be a prefix ofa channel name where the name space and channel name are separated by adot. In some implementations, name spaces can be used when specifyingchannel authorization settings. For instance, the messaging system 100may have app1.foo and app1.system.notifications channels where “app1” isthe name of the name space. The system can allow clients to subscribeand publish to the app1.foo channel. However, clients can only subscribeto, but not publish to the app1.system.notifications channel.

FIG. 1B illustrates functional layers of software on an example clientdevice. A client device (e.g., client 102) is a data processingapparatus such as, for example, a personal computer, a laptop computer,a tablet computer, a smart phone, a smart watch, or a server computer.Other types of client devices are possible. The application layer 104comprises the end-user application(s) that will integrate with thePubSub system 100. The messaging layer 106 is a programmatic interfacefor the application layer 104 to utilize services of the system 100 suchas channel subscription, message publication, message retrieval, userauthentication, and user authorization. In some implementations, themessages passed to and from the messaging layer 106 are encoded asJavaScript Object Notation (JSON) objects. Other message encodingschemes are possible.

The operating system 108 layer comprises the operating system softwareon the client 102. In various implementations, messages can be sent andreceived to/from the system 100 using persistent or non-persistentconnections. Persistent connections can be created using, for example,network sockets. A transport protocol such as TCP/IP layer 112implements the Transport Control Protocol/Internet Protocolcommunication with the system 100 that can be used by the messaginglayer 106 to send messages over connections to the system 100. Othercommunication protocols are possible including, for example, UserDatagram Protocol (UDP). In further implementations, an optionalTransport Layer Security (TLS) layer 110 can be employed to ensure theconfidentiality of the messages.

FIG. 2 is a diagram of an example messaging system 100. The system 100provides functionality for implementing PubSub communication patterns.The system comprises software components and storage that can bedeployed at one or more data centers 122 in one or more geographiclocations, for example. The system comprises MX nodes (e.g., MX nodes ormultiplexer nodes 202, 204 and 206), Q nodes (e.g., Q nodes or queuenodes 208, 210 and 212), one or more channel manager nodes (e.g.,channel managers 214, 215), and optionally one or more C nodes (e.g., Cnodes or cache nodes 220 and 222). Each node can execute in a virtualmachine or on a physical machine (e.g., a data processing apparatus).Each MX node serves as a termination point for one or more publisherand/or subscriber connections through the external network 216. Theinternal communication among MX nodes, Q nodes, C nodes, and the channelmanager, is conducted over an internal network 218, for example. By wayof illustration, MX node 204 can be the terminus of a subscriberconnection from client 102. Each Q node buffers channel data forconsumption by the MX nodes. An ordered sequence of messages publishedto a channel is a logical channel stream. For example, if three clientspublish messages to a given channel, the combined messages published bythe clients comprise a channel stream. Messages can be ordered in achannel stream by time of publication by the client, by time of receiptby an MX node, or by time of receipt by a Q node. Other ways forordering messages in a channel stream are possible. In the case wheremore than one message would be assigned to the same position in theorder one of the messages can be chosen (e.g., randomly) to have a latersequence in the order. Each channel manager node is responsible formanaging Q node load by splitting channel streams into so-calledstreamlets. Streamlets are discussed further below. The optional C nodesprovide caching and load removal from the Q nodes.

In the example messaging system 100, one or more client devices(publishers and/or subscribers) establish respective persistentconnections (e.g., TCP connections) to an MX node (e.g., MX 204). The MXnode serves as a termination point for these connections. For instance,external messages (e.g., between respective client devices and the MXnode) carried by these connections can be encoded based on an externalprotocol (e.g., JSON). The MX node terminates the external protocol andtranslates the external messages to internal communication, and viceversa. The MX nodes publish and subscribe to streamlets on behalf ofclients. In this way, an MX node can multiplex and merge requests ofclient devices subscribing for or publishing to the same channel, thusrepresenting multiple client devices as one, instead of one by one.

In the example messaging system 100, a Q node (e.g., Q node 208) canstore one or more streamlets of one or more channel streams. A streamletis a data buffer for a portion of a channel stream. A streamlet willclose to writing when its storage is full. A streamlet will close toreading and writing and be de-allocated when its time-to-live (TTL) hasexpired. By way of illustration, a streamlet can have a maximum size of1 MB and a TTL of three minutes. Different channels can have streamletslimited by different TTLs. For instance, streamlets in one channel canexist for up to three minutes, while streamlets in another channel canexist for up to 10 minutes. In various implementations, a streamletcorresponds to a computing process running on a Q node. The computingprocess can be terminated after the streamlet's TTL has expired, thusfreeing up computing resources (for the streamlet) back to the Q node,for example.

When receiving a publish request from a client device, an MX node (e.g.,MX 204) makes a request to a channel manager (e.g., channel manager 214)to grant access to a streamlet to write the message being published.Note, however, that if the MX node has already been granted write accessto a streamlet for the channel (and the channel has not been closed towriting) the MX node can write the message to that streamlet withouthaving to request a grant to access the streamlet. Once a message iswritten to a streamlet for a channel the message can be read by MX nodesand provided to subscribers of that channel.

Similarly, when receiving a channel subscription request from a clientdevice, an MX node makes a request to a channel manager to grant accessto a streamlet for the channel to read messages from. If the MX node hasalready been granted read access to a streamlet for the channel (and thechannel's TTL has not been closed to reading) the MX node can readmessages from the streamlet without having to request a grant to accessthe streamlet. The read messages can then be forwarded to client devicesthat have subscribed to the channel. In various implementations,messages read from streamlets are cached by MX nodes so that MX nodescan reduce the number of times needed to read from the streamlets.

By way of illustration, an MX node can request a grant from the channelmanager that allows the MX node to store a block of data into astreamlet on a particular Q node that stores streamlets of theparticular channel. Example streamlet grant request and grant datastructures are as follows:

StreamletGrantRequest = {    ″channel″: string( )    ″mode″: ″read″ |″write″    “position”: 0 } StreamletGrant = {    ″streamlet-id″:″abcdef82734987″,    ″limit-size″: 2000000, # 2 megabytes max   ″limit-msgs″: 5000, # 5 thousand messages max    ″limit-life″: 4000,# the grant is valid for 4 seconds    “q-node″: string( )    “position”:0 }

The StreamletGrantRequest data structure stores the name of the streamchannel and a mode indicating whether the MX node intends on readingfrom or writing to the streamlet. The MX node sends theStreamletGrantRequest to a channel manager node. The channel managernode, in response, sends the MX node a StreamletGrantResponse datastructure. The StreamletGrantResponse contains an identifier of thestreamlet (streamlet-id), the maximum size of the streamlet(limit-size), the maximum number of messages that the streamlet canstore (limit-msgs), the TTL (limit-life), and an identifier of a Q node(q-node) on which the streamlet resides. The StreamletGrantRequest andStreamletGrantResponse can also have a position field that points to aposition in a streamlet (or a position in a channel) for reading fromthe streamlet.

A grant becomes invalid once the streamlet has closed. For example, astreamlet is closed to reading and writing once the streamlet's TTL hasexpired and a streamlet is closed to writing when the streamlet'sstorage is full. When a grant becomes invalid, the MX node can request anew grant from the channel manager to read from or write to a streamlet.The new grant will reference a different streamlet and will refer to thesame or a different Q node depending on where the new streamlet resides.

FIG. 3A is a data flow diagram of an example method for writing data toa streamlet in various embodiments. In FIG. 3A, when an MX node (e.g.,MX 202) request to write to a streamlet is granted by a channel manager(e.g., channel manager 214), as described before, the MX nodeestablishes a Transmission Control Protocol (TCP) connection with the Qnode identified in the grant response received from the channel manager(302). A streamlet can be written concurrently by multiple write grants(e.g., for messages published by multiple publisher clients). Othertypes of connection protocols between the MX node and the Q node arepossible.

The MX node then sends a prepare-publish message with an identifier of astreamlet that the MX node wants to write to the Q node (304). Thestreamlet identifier and Q node identifier can be provided by thechannel manager in the write grant as described earlier. The Q nodehands over the message to a handler process 301 (e.g., a computingprocess running on the Q node) for the identified streamlet (306). Thehandler process can send to the MX node an acknowledgement (308). Afterreceiving the acknowledgement, the MX node starts writing (publishing)messages (e.g., 310, 312, 314, and 318) to the handler process, which inturns stores the received data in the identified streamlet. The handlerprocess can also send acknowledgements (316, 320) to the MX node for thereceived data. In some implementations, acknowledgements can bepiggy-backed or cumulative. For instance, the handler process can sendto the MX node an acknowledgement for every predetermined amount of datareceived (e.g., for every 100 messages received), or for everypredetermined time period (e.g., for every one millisecond). Otheracknowledgement scheduling algorithms such as Nagle's algorithm can beused.

If the streamlet can no longer accept published data (e.g., when thestreamlet is full), the handler process sends a Negative-Acknowledgement(NAK) message (330) indicating a problem, following by an EOF(end-of-file) message (332). In this way, the handler process closes theassociation with the MX node for the publish grant. The MX node can thenrequest a write grant for another streamlet from a channel manager ifthe MX node has additional messages to store.

FIG. 3B is a data flow diagram of an example method for reading datafrom a streamlet in various embodiments. In FIG. 3B, an MX node (e.g.,MX 202) sends to a channel manager (e.g., channel manager 214) a requestfor reading a particular channel starting from a particular message ortime offset in the channel. The channel manager returns to the MX node aread grant including an identifier of a streamlet containing theparticular message, a position in the streamlet corresponding to theparticular message, and an identifier of a Q node (e.g., Q node 208)containing the particular streamlet. The MX node then establishes a TCPconnection with the Q node (352). Other types of connection protocolsbetween the MX node and the Q node are possible.

The MX node then sends to the Q node a subscribe message with theidentifier of the streamlet (in the Q node) and the position in thestreamlet that the MX node wants to read from (356). The Q node handsover the subscribe message to a handler process 351 for the streamlet(356). The handler process can send to the MX node an acknowledgement(358). The handler process then sends messages (360, 364, 366), startingat the position in the streamlet, to the MX node. In someimplementations, the handler process can send all of the messages in thestreamlet to the MX node. After sending the last message in a particularstreamlet, the handler process can send a notification of the lastmessage to the MX node. The MX node can send to the channel manageranother request for another streamlet containing a next message in theparticular channel.

If the particular streamlet is closed (e.g., after its TTL has expired),the handler process can send an unsubscribe message (390), followed byan EOF message (392), to close the association with the MX node for theread grant. The MX node can close the association with the handlerprocess when the MX node moves to another streamlet for messages in theparticular channel (e.g., as instructed by the channel manager). The MXnode can also close the association with the handler process if the MXnode receives an unsubscribe message from a corresponding client device.

In various implementations, a streamlet can be written into and readfrom at the same time instance. For instance, there can be a valid readgrant and a valid write grant at the same time instance. In variousimplementations, a streamlet can be read concurrently by multiple readgrants (e.g., for channels subscribed to by multiple publisher clients).The handler process of the streamlet can order messages from concurrentwrite grants based on, e.g., time-of-arrival, and store the messagesbased on the order. In this way, messages published to a channel frommultiple publisher clients can be serialized and stored in a streamletof the channel.

In the messaging system 100, one or more C nodes (e.g., C node 220) canoffload data transfers from one or more Q nodes. For instance, if thereare many MX nodes requesting streamlets from Q nodes for a particularchannel, the streamlets can be offloaded and cached in one or more Cnodes. The MX nodes (e.g., as instructed by read grants from a channelmanager) can read the streamlets from the C nodes instead.

As described above, messages for a channel in the messaging system 100are ordered in a channel stream. A channel manager (e.g., channelmanager 214) splits the channel stream into fixed-sized streamlets thateach reside on a respective Q node. In this way, storing a channelstream can be shared among many Q nodes; each Q node stores a portion(one or more streamlets) of the channel stream. More particularly, astreamlet can be stored in registers and dynamic memory elementsassociated with a computing process on a Q node thus avoiding the needto access persistent, slower storage devices such as hard disks. Thisresults in faster message access. The channel manager can also balanceload among Q nodes in the messaging system 100 by monitoring respectiveworkload of the Q nodes and allocating streamlets in a way that avoidsoverloading any one Q node.

In various implementations, a channel manager maintains a listidentifying each active streamlet, the respective Q node on which thestreamlet resides, and identification of the position of the firstmessage in the streamlet, and whether the streamlet is closed forwriting. In some implementations, Q nodes notify the channel manager andany MX nodes that are publishing to a streamlet the streamlet is closeddue to being full or when the streamlet's TTL has expired. When astreamlet is closed the streamlet remains on the channel manager's listof active streamlets until the streamlet's TTL has expired so that MXnodes can continue to retrieve messages from the streamlet.

When an MX node requests a write grant for a given channel and there isnot a streamlet for the channel that can be written to, the channelmanager allocates a new streamlet on one of the Q nodes and returns theidentity of the streamlet and the Q node in the StreamletGrant.Otherwise the channel manager returns the identity of the currently openfor writing streamlet and corresponding Q node in the StreamletGrant. MXnodes can publish messages to the streamlet until the streamlet is fullor the streamlet's TTL has expired, after which a new streamlet can beallocated by the channel manager.

When an MX node requests a read grant for a given channel and there isnot a streamlet for the channel that can be read from, the channelmanager allocates a new streamlet on one of the Q nodes and returns theidentity of the streamlet and the Q node in the StreamletGrant.Otherwise, the channel manager returns the identity of the streamlet andQ node that contains the position from which the MX node wishes to read.The Q node can then begin sending messages to the MX node from thestreamlet beginning at the specified position until there are no moremessages in the streamlet to send. When a new message is published to astreamlet, MX nodes that have subscribed to that streamlet will receivethe new message. If a streamlet's TTL has expired the handler 351 sendsEOF message (392) to any MX nodes that are subscribed to the streamlet.

As described earlier in reference to FIG. 2, the messaging system 100can include multiple channel managers (e.g., channel managers 214, 215).Multiple channel managers provide resiliency and prevent single point offailure. For instance, one channel manager can replicate lists ofstreamlets and current grants it maintains to another “slave” channelmanager. As for another example, multiple channel managers cancoordinate operations between them using distributed consensus protocolssuch as Paxos or Raft protocols.

FIG. 4A is a data flow diagram of an example method for publishingmessages to a channel of a messaging system. In FIG. 4A, publishers(e.g., publisher clients 402, 404, 406) publish messages to themessaging system 100 described earlier in reference to FIG. 2. Forinstance, publishers 402 respectively establish connections 411 and sendpublish requests to the MX node 202. Publishers 404 respectivelyestablish connections 413 and send publish requests to the MX node 206.Publishers 406 respectively establish connections 415 and send publishrequests to the MX 204. Here, the MX nodes can communicate (417) with achannel manager (e.g., channel manager 214) and one or more Q nodes(e.g., Q nodes 212 and 208) in the messaging system 100 via the internalnetwork 218.

By way of illustration, each publish request (e.g., in JSON key/valuepairs) from a publisher to an MX node includes a channel name and amessage. The MX node (e.g., MX 202) can assign the message in thepublish request to a distinct channel in the messaging system 100 basedon the channel name (e.g., “foo”) of the publish request. The MX nodecan confirm the assigned channel with the channel manager 214. If thechannel (specified in the subscribe request) does not yet exist in themessaging system 100, the channel manager can create and maintain a newchannel in the messaging system 100. For instance, the channel managercan maintain a new channel by maintaining a list identifying each activestreamlet of the channel's stream, the respective Q node on which thestreamlet resides, and identification of the positions of the first andlast messages in the streamlet as described earlier.

For messages of a particular channel, the MX node can store the messagesin one or more buffers or streamlets in the messaging system 100. Forinstance, the MX node 202 receives from the publishers 402 requests topublish messages M11, M12, M13, and M14 to a channel foo. The MX node206 receives from the publishers 404 requests to publish messages M78and M79 to the channel foo. The MX node 204 receives from the publishers406 requests to publish messages M26, M27, M28, M29, M30, and M31 to thechannel foo.

The MX nodes can identify one or more streamlets for storing messagesfor the channel foo. As described earlier, each MX node can request awrite grant from the channel manager 214 that allows the MX node tostore the messages in a streamlet of the channel foo. For instance, theMX node 202 receives a grant from the channel manager 214 to writemessages M11, M12, M13, and M14 to a streamlet 4101 on the Q node 212.The MX node 206 receives a grant from the channel manager 214 to writemessages M78 and M79 to the streamlet 4101. Here, the streamlet 4101 isthe last one (at the moment) of a sequence of streamlets of the channelstream 430 storing messages of the channel foo. The streamlet 4101 hasmessages (421) of the channel foo that were previously stored in thestreamlet 4101, but is still open, i.e., the streamlet 4101 still hasspace for storing more messages and the streamlet's TTL has not expired.

The MX node 202 can arrange the messages for the channel foo based onthe respective time that each message was received by the MX node 202,e.g., M11, M13, M14, M12 (422), and store the received messages asarranged in the streamlet 4101. That is, the MX node 202 receives M11first, followed by M13, M14, and M12. Similarly, the MX node 206 canarrange the messages for the channel foo based on their respective timethat each message was received by the MX node 206, e.g., M78, M79 (423),and store the received messages as arranged in the streamlet 4101.

The MX node 202 (or MX node 206) can store the received messages usingthe method for writing data to a streamlet described earlier inreference to FIG. 3A, for example. In various implementations, the MXnode 202 (or MX node 206) can buffer (e.g., in a local data buffer) thereceived messages for the channel foo and store the received messages ina streamlet for the channel foo (e.g., streamlet 4101) when the bufferedmessages reaches a predetermined size (e.g., 100 messages), or when apredetermined time (e.g., 50 milliseconds) has elapsed. That is, the MXnode 202 can store in the streamlet 100 messages a time or in every 50milliseconds. Other acknowledgement scheduling algorithms such asNagle's algorithm can be used.

In various implementations, the Q node 212 (e.g., a handler process)stores the messages of the channel foo in the streamlet 4101 in theorder as arranged by the MX node 202 and MX node 206. The Q node 212stores the messages of the channel foo in the streamlet 4101 in theorder the Q node 212 receives the messages. For instance, assuming thatthe Q node 212 receives messages M78 (from the MX node 206) first,followed by messages M11 and M13 (from the MX node 202), M79 (from theMX node 206), M14, M12, and M26 (from the MX node 202). The Q node 212stores in the streamlet 4101 the messages in the order as received,e.g., M78, M11, M13, M79, M14, and M12, immediately after the messages421 that are already stored in the streamlet 4101. In this way, messagespublished to the channel foo from multiple publishers (e.g., 402, 404)can be serialized in a particular order and stored in the streamlet 4101of the channel foo. Different subscribers subscribe to the channel foowill receive messages of the channel foo in the same particular order,as will be described in more detail in reference to FIG. 4B.

In the example of FIG. 4A, at a time instance after the message M12 wasstored in the streamlet 4101, the MX node 204 requests a grant from thechannel manager 214 to write to the channel foo. The channel manager 214provides the MX node 204 a grant to write messages to the streamlet4101, as the streamlet 4101 is still open for writing. The MX node 204arranges the messages for the channel foo based on their respective timethat each message was received by the MX node 204, e.g., M26, M27, M31,M29, M30, M28 (424), and stores the messages as arranged for the channelfoo.

By way of illustration, assume that the message M26 is stored to thelast available position of the streamlet 4101. As the streamlet 4101 isnow full, the Q node 212 sends to the MX node 204 a NAK message,following by an EOF message, to close the association with the MX node204 for the write grant, as described earlier in reference to FIG. 3A.The MX node 204 then requests another write grant from the channelmanager 214 for additional messages (e.g., M27, M31, and so on) for thechannel foo.

The channel manager 214 can monitor available Q nodes in the messagingsystem 100 for their respective workloads (e.g., how many streamlets areresiding in each Q node). The channel manager 214 can allocate astreamlet for the write request from the MX node 204 such thatoverloading (e.g., too many streamlets or too many read or write grants)can be avoided for any given Q node. For instance, the channel manager214 can identify a least loaded Q node in the messaging system 100 andallocate a new streamlet on the least loaded Q node for write requestsfrom the MX node 204. In the example of FIG. 4A, the channel manager 214allocates a new streamlet 4102 on the Q node 208 and provides a writegrant to the MX node 204 to write messages for the channel foo to thestreamlet 4102. As shown in FIG. 4A, the Q node stores in the streamlet4102 the messages from the MX node 204 in an order as arranged by the MXnode 204: M27, M31, M29, M30, and M28 (assuming that there is no otherconcurrent write grant for the streamlet 4102 at the moment).

When the channel manager 214 allocates a new streamlet (e.g., streamlet4102) for a request for a grant from an MX node (e.g., MX node 204) towrite to a channel (e.g., foo), the channel manager 214 assigns to thestreamlet its TTL that will expire after TTLs of other streamlets thatare already in the channel's stream. For instance, the channel manager214 can assign to each streamlet of the channel foo's stream a TTL of 3minutes when allocating the streamlet. That is, each streamlet willexpire 3 minutes after it's allocated (created) by the channel manager214. Since a new streamlet is allocated after a previous streamlet isclosed (e.g., filled entirely or expired), in this way, the channelfoo's stream comprises streamlets that each expires sequentially afterits previous streamlet expires. For instance, as shown in an examplestream 430 of the channel foo in FIG. 4A, streamlet 4098 and streamletsbefore 4098 have expired (as indicated by the dotted-lined gray-outboxes). Messages stored in these expired streamlets are not availablefor reading for subscribers of the channel foo. Streamlets 4099, 4100,4101, and 4102 are still active (not expired). The streamlets 4099,4100, and 4101 are closed for writing, but still are available forreading. The streamlet 4102 is available for reading and writing, at themoment when the message M28 was stored in the streamlet 4102. At a latertime, the streamlet 4099 will expire, following by the streamlet 4100,4101, and so on.

FIG. 4B is a data flow diagram of an example method for subscribing to achannel of a messaging system. In FIG. 4B, a subscriber 480 establishesa connection 462 with an MX node 461 of the messaging system 100.Subscriber 482 establishes a connection 463 with the MX node 461.Subscriber 485 establishes a connection 467 with an MX node 468 of themessaging system 100. Here, the MX nodes 461 and 468 can respectivelycommunicate (464) with the channel manager 214 and one or more Q nodesin the messaging system 100 via the internal network 218.

A subscriber (e.g., subscriber 480) can subscribe to the channel foo ofthe messaging system 100 by establishing a connection (e.g., 462) andsending a request for subscribing to messages of the channel foo to anMX node (e.g., MX node 461). The request (e.g., in JSON key/value pairs)can include a channel name “foo.” When receiving the subscribe request,the MX node 461 can send to the channel manager 214 a request for a readgrant for a streamlet in the channel foo's channel stream.

By way of illustration, assume that at the current moment the channelfoo's channel stream 431 includes active streamlets 4102, 4103, and4104, as shown in FIG. 4B. The streamlets 4102 and 4103 each are full.The streamlet 4104 stores messages of the channel foo, including thelast message (at the current moment) stored at a position 47731.Streamlets 4101 and streamlets before 4101 are invalid, as theirrespective TTLs have expired. Note that the messages M78, M11, M13, M79,M14, M12, and M26 stored in the streamlet 4101, described earlier inreference to FIG. 4A, are no longer available for subscribers of thechannel foo, since the streamlet 4101 is no longer valid, as its TTL hasexpired. As described earlier, each streamlet in the channel foo'sstream has a TTL of 3 minutes, thus only messages (as stored instreamlets of the channel foo) that are published to the channel foo(i.e., stored into the channel's streamlets) no earlier than 3 minutesfrom the current time can be available for subscribers of the channelfoo.

The MX node 461 can request a read grant for all available messages inthe channel foo, for example, when the subscriber 480 is a newsubscriber to the channel foo. Based on the request, the channel manager214 provides the MX node 461 a read grant to the streamlet 4102 (on theQ node 208) that is the earliest streamlet in the active streamlets ofthe channel foo (i.e., the first in the sequence of the activestreamlets). The MX node 461 can retrieve messages in the streamlet 4102from the Q node 208, using the method for reading data from a streamletdescribed earlier in reference to FIG. 3B, for example. Note that themessages retrieved from the streamlet 4102 maintain the same order asstored in the streamlet 4102. In various implementations, when providingmessages stored in the streamlet 4102 to the MX node 461, the Q node 208can buffer (e.g., in a local data buffer) the messages and sends themessages to the MX node 461 when the buffer messages reaches apredetermined size (e.g., 200 messages), or a predetermined time (e.g.,50 milliseconds) has elapsed. That is, the Q node 208 can send thechannel foo's messages (from the streamlet 4102) to the MX node 461 200messages a time, or in every 50 milliseconds. Other acknowledgementscheduling algorithms such as Nagle's algorithm can be used.

After receiving the last message in the streamlet 4102, the MX node 461can send an acknowledgement to the Q node 208, and send to the channelmanager 214 another request (e.g., for a read grant) for the nextstreamlet in the channel stream of the channel foo. Based on therequest, the channel manager 214 provides the MX node 461 a read grantto the streamlet 4103 (on Q node 472) that logically follows thestreamlet 4102 in the sequence of active streamlets of the channel foo.The MX node 461 can retrieve messages stored in the streamlet 4103,e.g., using the method for reading data from a streamlet describedearlier in reference to FIG. 3B, until it retrieves the last messagestored in the streamlet 4103. The MX node 461 can send to the channelmanager 214 yet another request for a read grant for messages in thenext streamlet 4104 (on Q node 474). After receiving the read grant, theMX node 461 retrieves message of the channel foo stored in the streamlet4104, until the last message at the position 47731. Similarly, the MXnode 468 can retrieve messages from the streamlets 4102, 4103, and 4104(as shown with dotted arrows in FIG. 4B), and provide the messages tothe subscriber 485.

The MX node 461 can send the retrieved messages of the channel foo tothe subscriber 480 (via the connection 462) while receiving the messagesfrom the Q node 208, 472, or 474. In various implementations, the MXnode 461 can store the retrieved messages in a local buffer. In thisway, the retrieved messages can be provided to another subscriber (e.g.,subscriber 482) when the other subscriber subscribes to the channel fooand requests the channel's messages. The MX node 461 can remove messagesstored in the local buffer that each has a time of publication that hasexceeded a predetermined time period. For instance, the MX node 461 canremove messages (stored in the local buffer) with respective times ofpublication exceeding 3 minutes. In some implementations, thepredetermined time period for keeping messages in the local buffer on MXnode 461 can be the same as or similar to the time-to-live duration of astreamlet in the channel foo's channel stream, since at a given moment,messages retrieved from the channel's stream do not include those instreamlets having respective time-to-lives that had already expired.

The messages retrieved from the channel stream 431 and sent to thesubscriber 480 (by the MX node 461) are arranged in the same order asthe messages were stored in the channel stream. For instance, messagespublished to the channel foo are serialized and stored in the streamlet4102 in a particular order (e.g., M27, M31, M29, M30, and so on), thenstored subsequently in the streamlet 4103, and the streamlet 4104. TheMX node retrieves messages from the channel stream 431 and provides theretrieved messages to the subscriber 480 in the same order as themessages are stored in the channel stream: M27, M31, M29, M30, and soon, followed by ordered messages in the streamlet 4103, and followed byordered messages in the streamlet 4104.

Instead of retrieving all available messages in the channel stream 431,the MX node 461 can request a read grant for messages stored in thechannel stream 431 starting from a message at particular position, e.g.,position 47202. For instance, the position 47202 can correspond to anearlier time instance (e.g., 10 seconds before the current time) whenthe subscriber 480 was last subscribing to the channel foo (e.g., via aconnection to the MX node 461 or another MX node of the messaging system100). The MX node 461 can send to the channel manager 214 a request fora read grant for messages starting at the position 47202. Based on therequest, the channel manager 214 provides the MX node 461 a read grantto the streamlet 4104 (on the Q node 474) and a position on thestreamlet 4104 that corresponds to the channel stream position 47202.The MX node 461 can retrieve messages in the streamlet 4104 startingfrom the provided position, and send the retrieved messages to thesubscriber 480.

As described above in reference to FIGS. 4A and 4B, messages publishedto the channel foo are serialized and stored in the channel's streamletsin a particular order. The channel manager 214 maintains the orderedsequence of streamlets as they are created throughout their respectivetime-to-lives. Messages retrieved from the streamlets by an MX node(e.g., MX node 461, or MX node 468) and provided to a subscriber can be,in some implementations, in the same order as the messages are stored inthe ordered sequence of streamlets. In this way, messages sent todifferent subscribers (e.g., subscriber 480, subscriber 482, orsubscriber 485) can be in the same order (as the messages are stored inthe streamlets), regardless which MX nodes the subscribers are connectedto.

In various implementations, a streamlet stores messages in a set ofblocks of messages. Each block stores a number of messages. Forinstance, a block can store two hundred kilobytes of messages. Eachblock has its own time-to-live, which can be shorter than thetime-to-live of the streamlet holding the block. Once a block's TTL hasexpired, the block can be discarded from the streamlet holding theblock, as described in more detail below in reference to FIG. 4C.

FIG. 4C is an example data structure for storing messages of a channelof a messaging system. As described with the channel foo in reference toFIGS. 4A and 4B, assume that at the current moment the channel foo'schannel stream 432 includes active streamlets 4104 and 4105, as show inFIG. 4C. Streamlet 4103 and streamlets before 4103 are invalid, as theirrespective TTLs have expired. The streamlet 4104 is already full for itscapacity (e.g., as determined by a corresponding write grant) and isclosed for additional message writes. The streamlet 4104 is stillavailable for message reads. The streamlet 4105 is open and is availablefor message writes and reads.

By way of illustration, the streamlet 4104 (e.g., a computing processrunning on the Q node 474 shown in FIG. 4B) currently holds two blocksof messages. Block 494 holds messages from channel positions 47301 to47850. Block 495 holds messages from channel positions 47851 to 48000.The streamlet 4105 (e.g., a computing process running on another Q nodein the messaging system 100) currently holds two blocks of messages.Block 496 holds messages from channel positions 48001 to 48200. Block497 holds messages starting from channel position 48201, and stillaccepts additional messages of the channel foo.

When the streamlet 4104 was created (e.g., by a write grant), a firstblock (sub-buffer) 492 was created to store messages, e.g., from channelpositions 47010 to 47100. Later on, after the block 492 had reached itscapacity, another block 493 was created to store messages, e.g., fromchannel positions 47111 to 47300. Blocks 494 and 495 were subsequentlycreated to store additional messages. Afterwards, the streamlet 4104 wasclosed for additional message writes, and the streamlet 4105 was createdwith additional blocks for storing additional messages of the channelfoo.

In this example, the respective TTL's of blocks 492 and 493 had expired.The messages stored in these two blocks (from channel positions 47010 to47300) are no longer available for reading by subscribers of the channelfoo. The streamlet 4104 can discard these two expired blocks, e.g., byde-allocating the memory space for the blocks 492 and 493. The blocks494 or 495 could become expired and be discarded by the streamlet 4104,before the streamlet 4104 itself become invalid. Or streamlet 4104itself could become invalid before the blocks 494 or 495 become expired.In this way, a streamlet can hold one or more blocks of messages, orcontain no block of messages, depending respective TTLs of the streamletand blocks, for example.

A streamlet, or a computing process running on a Q node in the messagingsystem 100, can create a block for storing messages of a channel byallocating a certain size of memory space from the Q node. The streamletcan receive, from an MX node in the messaging system 100, one message atime and store the received message in the block. Alternatively, the MXnode can assemble (i.e., buffer) a group of messages and send the groupof messages to the Q node. The streamlet can allocate a block of memoryspace (from the Q node) and stored the group of messages in the block.The MX node can also perform compression on the group of messages, e.g.,by removing a common header from each message.

FIG. 5 is a flowchart of an example method for publishing andsubscribing messages for publishers and subscribers of a messagingsystem. The method can be implemented using an MX node (e.g., MX node204, MX 461) and a Q node (e.g., Q node 212, Q node 208) of themessaging system 100, for example. The method begins by receiving from aplurality of publisher clients a plurality of messages (e.g., MX node204; Step 502). The method assigns each of the messages to one of aplurality of distinct channels wherein each channel comprises an orderedplurality of messages (e.g., MX node 204, Step 504). The method storesmessages of each of the channels in one or more respective buffersaccording to the order, each buffer having a respective time-to-live(e.g., Q node 208, Step 506). The method retrieves messages for one ormore of the channels from respective buffers having time-to-lives thathave not expired and according to the order (e.g., MX node 461; Step508). The method sends the retrieved messages to a plurality ofsubscriber clients (e.g., MX node 461, Step 510).

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. Alternatively or inaddition, the program instructions can be encoded on anartificially-generated propagated signal, e.g., a machine-generatedelectrical, optical, or electromagnetic signal, that is generated toencode information for transmission to suitable receiver apparatus forexecution by a data processing apparatus. A computer storage medium canbe, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. Moreover,while a computer storage medium is not a propagated signal, a computerstorage medium can be a source or destination of computer programinstructions encoded in an artificially-generated propagated signal. Thecomputer storage medium can also be, or be included in, one or moreseparate physical components or media (e.g., multiple CDs, disks, orother storage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative,procedural, or functional languages, and it can be deployed in any form,including as a stand-alone program or as a module, component,subroutine, object, or other unit suitable for use in a computingenvironment. A computer program may, but need not, correspond to a filein a file system. A program can be stored in a portion of a file thatholds other programs or data (e.g., one or more scripts stored in amarkup language resource), in a single file dedicated to the program inquestion, or in multiple coordinated files (e.g., files that store oneor more modules, sub-programs, or portions of code). A computer programcan be deployed to be executed on one computer or on multiple computersthat are located at one site or distributed across multiple sites andinterconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic, magneto-optical disks, or optical disks.However, a computer need not have such devices. Moreover, a computer canbe embedded in another device, e.g., a smart phone, a mobile audio orvideo player, a game console, a Global Positioning System (GPS)receiver, or a portable storage device (e.g., a universal serial bus(USB) flash drive), to name just a few. Devices suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending resources to and receiving resources from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back-end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front-end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described in this specification, or any combination of one ormore such back-end, middleware, or front-end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), andpeer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data (e.g., an HTML page) to a clientdevice (e.g., for purposes of displaying data to and receiving userinput from a user interacting with the client device). Data generated atthe client device (e.g., a result of the user interaction) can bereceived from the client device at the server.

A system of one or more computers can be configured to performparticular operations or actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform the actions. One or morecomputer programs can be configured to perform particular operations oractions by virtue of including instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the actions.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. A computer-implemented method, comprising:receiving a plurality of messages from a plurality of publisher clients,each message associated with a particular one of a plurality ofchannels; and storing each of the received messages in a respectivebuffer for the channel associated with the message and according to anorder, the buffer having an expiration time based on when the buffer wasallocated to the channel, wherein retrieval of messages for theparticular channel is from one or more buffers allocated to the channelthat have not expired and according to the order.
 2. The method of claim1, wherein storing each of the received messages comprises: identifyinga first buffer for storing messages of the particular channel; orderingmessages for the particular channel based on, at least in part, arespective time each of the messages was received from respectivepublisher clients; and storing a first portion of the messages of theparticular channel as ordered in the first buffer.
 3. The method ofclaim 2, further comprising: determining that the first buffer is fulland, based thereon: identifying a second buffer; and storing a differentsecond portion of the messages of the particular channel as ordered inthe second buffer.
 4. The method of claim 2, wherein storing the firstportion of the messages further comprises: buffering one or more of thefirst portion of the messages in a local data buffer; and storing thebuffered messages in the first buffer after a number of bufferedmessages exceeds a predetermined size or after a predetermined timeperiod has elapsed.
 5. The method of claim 1, wherein the particularchannel is initiated by a message from a publisher client.
 6. The methodof claim 1, comprising: retrieving messages for the particular channelby initiating a connection from a subscriber client subscribing to theparticular channel
 7. The method of claim 6, wherein retrieving messagescomprises: retrieving messages for the particular channel and storingthe retrieved messages in a local buffer, wherein each message isremoved from the local buffer when a respective time of publication ofthe message has exceeded a predetermined time period.
 8. The method ofclaim 6, comprising: sending messages retrieved for the particularchannel to one or more subscriber clients that have subscribed to theparticular channel.
 9. The method of claim 1, wherein storing each ofthe received messages in the respective buffer for the associatedchannel and according to the order comprises: storing messages of eachchannel earlier in the order in buffers having time-to-lives that willexpire sooner than buffers used to store messages of each channel laterin the order.
 10. The method of claim 1, wherein the order comprises theorder in which messages for the channel were received.
 11. A systemcomprising: one or more computers programmed to perform operationscomprising: receiving a plurality of messages from a plurality ofpublisher clients, each message associated with a particular one of aplurality of channels; and storing each of the received messages in arespective buffer for the channel associated with the message andaccording to an order, the buffer having an expiration time based onwhen the buffer was allocated to the channel, wherein retrieval ofmessages for the particular channel is from one or more buffersallocated to the channel that have not expired and according to theorder.
 12. The system of claim 11, wherein storing each of the receivedmessages comprises: identifying a first buffer for storing messages ofthe particular channel; ordering messages for the particular channelbased on, at least in part, a respective time each of the messages wasreceived from respective publisher clients; and storing a first portionof the messages of the particular channel as ordered in the firstbuffer.
 13. The system of claim 12, wherein the operations furthercomprise: determining that the first buffer is full and, based thereon:identifying a second buffer; and storing a different second portion ofthe messages of the particular channel as ordered in the second buffer.14. The system of claim 12, wherein storing the first portion of themessages further comprises: buffering one or more of the first portionof the messages in a local data buffer; and storing the bufferedmessages in the first buffer after a number of buffered messages exceedsa predetermined size or after a predetermined time period has elapsed.15. The system of claim 11, wherein the particular channel is initiatedby a message from a publisher client.
 16. The system of claim 11,wherein the operations further comprise: retrieving messages for theparticular channel by initiating a connection from a subscriber clientsubscribing to the particular channel.
 17. The system of claim 16,wherein retrieving messages comprises: retrieving messages for theparticular channel and storing the retrieved messages in a local buffer,wherein each message is removed from the local buffer when a respectivetime of publication of the message has exceeded a predetermined timeperiod.
 18. The system of claim 16, wherein the operations furthercomprise: sending messages retrieved for the particular channel to oneor more subscriber clients that have subscribed to the particularchannel.
 19. The system of claim 11, wherein storing each of thereceived messages in the respective buffer for the associated channeland according to the order comprises: storing messages of each channelearlier in the order in buffers having time-to-lives that will expiresooner than buffers used to store messages of each channel later in theorder.
 20. An article comprising: a non-transitory machine-readablemedium having instructions stored thereon that when executed by one ormore computers causes the computers to perform operations comprising:receiving a plurality of messages from a plurality of publisher clients,each message associated with a particular one of a plurality ofchannels; and storing each of the received messages in a respectivebuffer for the channel associated with the message and according to anorder, the buffer having an expiration time based on when the buffer wasallocated to the channel, wherein retrieval of messages for theparticular channel is from one or more buffers allocated to the channelthat have not expired and according to the order.